Interest has recently arisen in developing wideband continuous-time filters for use in high-definition television (HDTV) systems, as well as within high-speed analog to digital converters (ADCs). High-speed, low distortion, and a wide linear tuning range are several of the desired characteristics of continuous-time filters suitable for employment in such applications.
One type of continuous-time filter is realized using operational transconductance amplifier-capacitance (i.e., OTA-C) filters. Wideband OTA-C filters require the availability of wideband operational transconductance amplifiers (OTAs), which may be implemented using bipolar junction (BJT) technology. Although BJT technology offers superior high-frequency performance, there exist few techniques for designing broadband bipolar OTA amplifiers, especially those designed using BJT transconductance capacitor (gm-c) configurations. In addition, the potential advantages afforded by OTAs implemented using, for example, BJT technologies have not as yet been realized due to the reluctance of circuit designers to include lateral PNP transistors of low cut-off frequency (f.sub.T) within the signal path. However, recent advances in complementary BJT processing techniques have provided an opportunity for more flexible circuit design.
In conventional bipolar OTAs, a linear gain response is produced to input signals within a selected input dynamic range by setting the gain of the input stage by adjusting the bias current therethrough. That is, in order to design an OTA capable of producing a linear output in response to large input signals, it is desired to set the gain of the input stage at a relatively high level by increasing bias current. However, a large degenerative emitter resistance is often included in the input stage in order to reduce the output distortion produced in response to selected differential input signals, which reduces the tuning range of the OTA. That is, such emitter degeneration reduces the maximum degree to which the gain of the input stage may be changed by altering bias current, and hence limits the dynamic range of input signals capable of being accommodated by the OTA. Although emitter degeneration leads to the desired operating characteristics of increased input impedance and reduced input capacitance, a need exists for an OTA capable of providing both a wide tuning range in addition to such desired operating characteristics.